Method of focusing electron microscopes



J. A. RAJCHMAN METHOD 0F FoCUsING ELECTRON MIcRoscoPEs May 2, 1944.

Filed April 50, 1942 Bnventor Cttorneg Patented May 2, 1944 METHOD or FocUsrNG ELEc'rnoN mcnoscorEs Jan A. Raiclrman, Philadelphia. Pa., assignor -to Radio Corporation of America, a corporation of Delaware Application April 30, 1942, Serial No. 441,167

4 Claims. (CL Z50-49.5)

This invention relates generally to electron microscopes and particularly to methods and means for focusing an electron beam in such microscopes to obtain maximum definition ln the reproduced image. For the purpose of illustration, the invention will be described with reference to its application tov a scanning microscope in which the specimen is explored or scanned by' an electron probe or beam, and wherein the primary electrons passing through a specimen, or the secondary electrons emanating from it, are used to` actuate a suitable integrating device such as a facsimile recorder. It isv known to those skilled in the art that the resolving power of an electron microscope of the scanning type depends solely on the sharpness of the focus of the electron beam at the point of contact with the specimen. Von Ardenne points out in his-U. S. Patent 2,257,774, granted on October 1, 1941, that optimum results are achieved when the diameter of the electron beam at the point of contact with the specimen is less than the wave length of light.

One problem which has caused considerable Y diiilculty in the operation of scanning microscopes isthat of determining whether or not the electron beam is of the smallest possible diameter at the point of contact with the specimen. Due to the customary slow scanning speed of such microscopes, substantially instantaneous means for indicating the focus of the electron beam are necessary in order that correction oi' the focus can be made at any time during the scanning interval.

The copending application of Richard L.; Snyder, Jr., Serial No. 396,099, led May 31, 1941, described and generally claims a method for indicating high frequency components in the signal output of a scanning microscope, and controlling the focus of the microscope to produce a maximum high frequency output which is indicative of optimum focus of the electron beam at the point of contact with the specimen. The instant invention is an improvement over the method described in the above mentioned Snyder application.

Accordingly, the principal object of the present invention is to provide an improved method of and means for determining quickly and easily whether or not the electron beam or probe of a scanning type microscope is properly focused. Another object of the invention is to provide an improved method of and means for iiltering out all but the high frequency components in the microscope signal output, indicating the level of such high frequency components, and adjusting the focus of the electron beam to the adjustment where the high frequency componentsP in the signal output are a maximum. The level of the high frequency components in the signal output may be indicated either by a cathode ray oscillograph or a. meter or other device for indicating power, current, or voltage depending upon the particular form of lter circuit used. The invention will be described by reference to the drawing of which Figure 1 is a schematic view of a scanning type of electron microscope as described and claimed by Richard L. Snyder, Jr., in his copending application Serial No.A 391- 188, filed April 30, 1941; Figures 2 and 3 are y diagrammatic views illustrative of the movement of the specimen with respect to primary elec- Y tron beams of different diameters; and Figures 4, 5, 6 and 'l are graphs of wave patterns indicative of dierent conditions of focus in the elecray oscilloscope O, a meter 2l for producing 5 on the inner end of which an object holder 1 containing an object or specimen (not shown) to be examined is mounted. Intermediate the cathode 3 and object holder 1 is an electron lens system including a first apertured plate 9, a first group of lens elements L, a second apertured plate Il, and a series of objective lens elements L', through all of which electrons pass, in an undeviating path, to the object under examination.

As taught by Von Ardenne, the diameter of the electron beam at the point at which it impinges the object should preferably be less than the wave length of light. The scanning movement necessary to a complete examination of every part of the object is provided by moving the object holder l with respect to the 'beam in a manner later described. Secondary electrons, released by impact of the steady primary beam on the moving object, travel in the return direction through the objective lens elements L' and impinge on a fluorescent target I3 which is prei!- erably provided on the wall surrounding the opening in the plate Il through which the primary beam passes on its way to the objective lens. A photosensitive amplier, for* example,

augmented electron current proportionate to the intensity of the secondary electron stream from the object. The output of the multiplier, in this case, serves to actuate the facsimile recorder R which is operated in synchronism with the object holder l to provide a permanent, enormously magnied image of the surface of the object thus scanned The distance the object is moved by thei scanning mechanism must, of course, be exceedingly small when a greatly magnified recorder image is desired. To achieve such small movement, mechanically, the lever 5 which supports the object holder l' may be pivoted close to the object end of the lever, and the driving force applied to its opposite or outer end. In an arrangement' wherein the specimen is mounted one-half inch from the fulcrum of the lever 5 and the driving force applied flve inches from the fulcrum, the resulting reduction of ten to one is satisfactory. In the illustrated arrangement, two dynamic-type loudspeaker motors MI and M2 are employed for imparting the requisite scanning movement to the object holder I; one motor (MI serving to provide the line-scanning movement and the other (M2), the framescanning movement..

The motors MI and M2 are actuated through suitable amplifiers AI and A2 by photo tubes TI and T2, respectively, which are illuminated by light from two lamps Bl and B2 controlled by rotating spiral shutters or cams CI and C2, which are driven through a mechanical coupling EI, E2 by the line and frame scanning mechanism of the recorder R.. Where the recording equipment employs an auxiliary amplifier (not shown) which requires a carrier frequency, such carrier may be introduced by interrupting the primary beam from the electron gun by impressing an alternating voltage of the desired frequency arid wave shape on the controlgrid of the gun, as indicated at G. A detector shunted by a suitable filter, indicated generally at Y, may be provided if desired to remove the carrier from the signal current supplied to the oscilloscope O. The Illament heating current supply is provided by a regulated, high frequency source F, which is turned on only during the time the beam is cut 0E by the carrier generator V. Thus, when the beam is on, no heating current, and hence no'disturbing magnetic field or potential, is present when electrons are being emitted.

As previously indicated, the cathode ray oscilloscope O may be employed for determining the condition of focus of the primary electron beam in the microscope S. To this end the horizontal defiecting plates (or coils) of the oscilloscope are shown connected as through a lead 2| to the amplifier AI which supplies the driving current for the line scanning mechanism of the microscope, and the vertical deilecting plates (or coils) are supplied with signal currents from the electron multiplier I through an extension 23a of the lead 23 which supplies signaling current to the recorder R and through a suitable high-pass components of the signal output. Ordinarily the oscilloscope will remain in circuit while the-recorder R is in operation so that any lack of focus which may develop during the recording interval may be observed and corrected forthwith. However, a suitable switch 28 may be provided. if desired, for disconnecting the recorder during the focusing tests.

The currents supplied to the vertical deilecting plates of the oscilloscope from the photosensitive multiplier I5, is proportional to the intensity of the secondary electron stream from the object under examination. Thus, if the primary beam or probe should fail to'strike the" probes of different diameters. In Fig. 2, if theA target 1 is moved slowly into the field of the larger primary beam X the secondary emission resulting from impact of the beam will exhibit a gradual rise, indicated by the slope of the oscillograph pattern P, Fig, 4, from zero to maximum as more and more of the primary beam is presented to the target. 0n the other hand, referring now to Figs. 3 and 5, when the primary beam XI is of substantially smaller diameter, the rise from zero current (just prior to the time the edge of the target contacts the periphery of the beam) to maximum current (when the full area of the beam impinges the target) as the target 'I is moved, at the same rate as before, into the beam XI, will be much more abrupt and may in fact comprise a square top wave similar to the one shown P' in Fig. 5. Since, as is well kown in the art, a square top wave has a large high. frequency content in its makeup it will now be apparent that the relative amount of high frequency components in the output current of a scanning type electron microscope provides a useful indication of the focus or pointsharpness of the beam. in the object plane subject to investigation.

As a practical matter the foregoing method of determining whether or not the beam or probe alter 25 which passes only the high frequency 75 in the microscope Sis properly focused, maybe carried out in several different ways either before or after the object to be examined is mounted upon the specimen holder. Thus if the holder itself be secondarily emissive the desired contrast may be achieved by moving an edge of the holder into or out of the beam. Alternatively,

if the emissive ratio of the holder under bombardment by the beam is less than or greater than that of the object per se an informative pattern may be achieved by successively moving the holder and the specimen into or out of the path of the beam. In any event the force required to move the holder during the testing interval may be supplied by .the motor MI which normally provides the line scanning movement. Since ordinarily this movement is not so great as'to move the holder out of the range of the beam the intensity of the driving current supplied by the amplifier Al may be altered to provide the desired degree of movement of the lever arm 5 to which the holder] is amxed. Alternatively, the adjustment may be made mechanically by moving the specimen-moving assembly, including the scanning motors, lever and lever fulcrum in the desired direction.

The condition of focus of the microscope primary beam may also, be ascertained in accordance with the invention without altering the normal arrangement or movement of the specimen with respect to the beam, simplyby observing the relative presence or absence of high frequency componentsin the output current of the microscope during normal operation, e. g., during a recording interval. Thus, with the oscilloscope connected to the output of the high-pass lter 25 in the manner indicated in Fig. 1, only the high frequency components of the signal derived from the multiplier l will provide vertical deection of the oscilloscope patternfandwill be manifest by sharp vertical undulations. indicated at P2,

Fig. 6, in the main pattern of the Wave. The absence of such sharp vertical undulations in the wave pattern of Fig. 7 is indicative of a lack of focus in the primary electron beam of the microscope. The focus of the electron beam is therefore adjusted to provide maximum vertical delection on the oscilloscope.

The focus of the beam may be corrected, when necessary, either by changing the relative potential distribution among the lens element of the electron lens system L, L', or by moving the object holder with respect to the focal spot or cross-over point of the electrons on the targetside of the objective lens L.

An alternative method, which may be substituted for or used in conjunction with the method using the oscilloscope 0,'is to measure the high frequency output power, voltage or current of the high-pass filter circuit 25 by means of a suitable meter 21 of conventional design suitably connected in the lter output circuit. With this latter arrangement the electron beam focus is adjusted until the meter 21 provides a maximum indication. y

I claim as my invention:

1. The method of adjusting the focus of an electron beam in an electron microscope having a signal output circuit in which the signal amplitude is a function of the electron image contrast including the steps of ltering said signals to pass only predetermined high frequency components thereof, indicating the output level of said ltered signals, and altering the effective diameter of said beam until said indicated level is a maximum.

2. The method described in claim 1 including recording said signal output level.

3. The method of adjusting the focus of au electron beam in an electron microscope having a signal output circuit in which the signal amplitude is a function of the electron image contrast, and a cathode ray oscillograph associated with said circuit including the steps of ltering said signals to pass only predetermined high frequency components thereof, applying said filtered signals to said oscillograph to indicate the output level of said filtered signals, and altering the eiective diameter of said beam until said oscillograph indication is a maximum.

4. The method described in claim 3 including recording said signal output level.

JAN A. RAJCHMAN. 

